Emerging Technologies for Iron and Steelmaking

The iron and steel industry has undergone a technological revolution in the last 40 years. In a relatively short time, the North American industry has observed the complete disappearance of basic open hearth processing, as well as the wide spread adoption of continuous casting and the near complete shift of long product production to the electric arc furnace sector. These and other developments have dramatically affected the way steel is made, the price, quality and range of products generated, and changed the basic structure of the industry. The same trends can be observed in other industrialized nations and are reflected in the global industry as well. Competitive forces and market globalization will continue to drive the development and adoption of new iron and steelmaking technologies well into the 21st century. Industry response to specific local and global technology drivers will likely result in both incremental improvements in existing technologies, and in major developments in several key areas including direct iron making and near net shape casting.

INTRODUCTION

The origin of ferrous alloy production can be traced back to as early as 2000 BC, when writings from ancient China and India made reference to manmade ferrous metals. By 1350 BC to 1100 BC, the production of ferrous metals from iron ore had spread to a wide geographic area.1 More than 3,000 years after this early beginning of the Iron Age, modern iron and steelmakers still use the same carbothermic process discovered by early ironmakers. However, this industry continues to develop incremental and entirely novel technology improvements that are more efficient, more productive, and cheaper than existing processes to produce high quality ferrous alloys with a wide range of properties and end uses.

Modern ironmaking and steelmaking is extremely intensive in material and energy usage as well as in capital requirements. The industry is also faced with a wide range of environmental concerns that are fundamentally related to the high energy requirements, material usage, and the byproducts associated with producing more than 725 million tonnes of steel per year worldwide. A highly competitive steel market, due in part to rapid technological change and accelerating market globalization, requires the modern steelmaker to be sensitive to customer demands in terms of product properties, quality, price, and delivery. Although it is the product of an extraordinary array of high-tech processes, steel is a raw material in the modern economy and is at the front end of many complex manufacturing chains. As a result, the steel producer is very sensitive to a dynamic market with periods of economic boom and slowdown. However, the steel industry is bridled with high fixed capital costs and processes, which are constrained to high production rates by efficiencies and economies of scale. A highly competitive world steel market has, in turn, produced an environment where capital resources are short and the cost of failed technological ventures dear. Therefore, the risk associated with being a technology leader is very high. In spite of this, the last 30 years have shown several times over that the technology of steelmaking can change rapidly on a global scale. The risk associated with being a technology follower can also prove to be painful.The Sloan Steel Industry Study, conducted by Carnegie Mellon University2, identified four main technology drivers for the steel industry: high capital costs, raw materials shortages, environmental concerns, and customer demands.

The American Iron and Steel Institute (AISI) combined the Carnegie Mellon study and input from a large number of industry experts to develop a technology roadmap3, a guide for future R&D efforts in North America. The AISI Roadmap is intended to focus attention on clear research needs of the industry. Although some of the technology needs and drivers identified in the AISI roadmap are specific to the North American industry, many of the identified research areas are the focus of worldwide research efforts and reflect universaltechnology needs for the industry. 4–6

ADVANCES IN IRON AND STEELMAKING

The blast furnace, in various forms, has remained the workhorse of world-wide virgin iron production for more than 200 years, producing carbon-saturated “hot metal” for subsequent processing by steelmaking processes. However, the modern blast furnace has advanced a long way from its earlier ancestors. Most modern large-capacity blast furnaces represent extremely efficient chemical reactors, capable of stable operation with an impressive range of reactant feed materials. The injection of pulverized coal, natural gas, oil, and, in some cases, recycled plastics to replace a portion of the metallurgical coke used as the primary reductant and source of chemical energy represents an important development in the process.

Coke is produced by baking coal in the absence of oxygen to remove the volatile hydrocarbons contained in coal. The resulting coke is mechanically strong, porous, and chemically reactive, which are all critical properties for stable blast furnace operation. In addition to supplying carbon for heat and the reduction of iron ore, coke must also physically support the burden in the blast furnace shaft and remain permeable to the hot air blast entering from the bottom. Coke-making is extremely problematic from an environmental perspective, as many of the hydrocarbons driven off during the coking process are hazardous. Also, not all types of coal are suitable for the production of coke. Recently, demand has decreased for the byproducts from coke-making for secondary processing into chemical products.3 In developed countries, aging coking facilities and tightening environmental control have made coke-making an economic liability. Therefore, decreasing both the coke rate and the over-all fuel rate of the blast furnace has been a major focus of recent developments. Figure 1 shows the evolution of blast furnace consumption of reductants in France in the last 30 years.

Similar trends can be observed in most developed countries. However, the relative proportions of coal, natural gas, and oil usage are dependant on several factors. These factors include local availability, fuel price, and the capital requirements of the injection equipment. Figure 2 shows the coke and coal consumption rates per ton of hot metal in Europe, Japan, and the United States. At high coal injection rates, partially combusted coal char builds up in the area near injector. This can lead to reduced gas permeability and currently sets the practical limit for coal injection. Extensive experimentation in the United States and elsewhere has found an optimum combination of fuels that allows for stable operation at low coke rates. That ideal mix, per ton of hot metal produced, includes metallurgical coke, 230 kg; nut coke, 40 kg; injected coal, 180 kg; and injected natural gas, 50 kg.

Improvements in process control and reduced refractory wear have increased blast furnace campaign life significantly, which is critical to the economics of the process. The current expected lifetime of newly rebuilt furnaces is 20 years or greater. Improved process control, burden design, regular maintenance, and reduction of unscheduled shut-downs has had a dramatic effect on the productivity of blast furnace operations.4–7 Many steel companies have shut down older furnaces while maintaining or increasing hot-metal production by increasing the productivity of newer furnaces.

In the past 5 to 10 years, there has been a rapid increase in the production of iron via direct reduction processes. This new production has been dominated by the gas-based Midrex and Hyl processes, although several new plants based on other processes have begun production. This additional world wide ironmaking capacity has primarily served the electric arc furnace industry, providing an alternative to high quality and expensive scrap as a source of clean, low residual iron units.

In the last 40 years, the basic open-hearth process has been almost completely replaced worldwide by various top, bottom, or combination blown basic oxygen steel making (OSM) processes.1 Since the adoption of basic oxygen steelmaking, continuous incremental improvements on the various forms of the process have improved the productivity and efficiency of oxygen steelmaking vessels. Development efforts have included experimentation with various combination top and bottom blowing configurations, natural gas shielding of bottom oxygen tuyeres, bottom stirring, top lance design, post combustion, slag formation control, process monitoring and control, and refractory design. A recent, important development in oxygen steelmaking has been the adoption of slag-splashing practices to increase furnace lining campaigns to more than 20,000 heats.3,8 In this practice, the furnace refractory lining is coated with slag between heats by nitrogen injection into the vessel after tapping of the liquid steel. Although the solidified slag coating eventually remelts in the subsequent heat or heats, this practice has effectively extended furnace lining life.

Figure 3. Evolution of steel by process from 1955 to 1996. Adapted from Fruehan.1 (Original source: International Iron andSteel Institute.)

The abandonment of open-hearth steelmaking practices for oxygen steel-making was accompanied by a parallel widespread departure from ingot casting and slabbing practices to the continuous casting of steel. The increases in productivity and yield associated with continuous casting have had a dramatic effect on the steel industry worldwide. From the mid- to late 1960s to present, the amount of continuously cast steel as a percentage of total steel production has risen from essentially 0% to more than 90% in most countries. Most of that change occurred in the relatively short period from 1970 to 1990.9 Critical research regarding the fundamentals of solidification, defect formation and modification, fluid flow in the continuous-caster mold and developing steel shell, refractory interactions, and mold flux design have led to improvements in the control and reliability of continuous-casting processes. In North America, this knowledge base was developed through the research efforts of individual steel companies and through significant contributions by such universities as Carnegie Mellon University, the Massachusetts Institute of Technology (MIT), The University of British Columbia, The University of Illinois, and others.

Because oxygen steelmaking processes melt less scrap than open-hearth steelmaking, the adoption of oxygen steelmaking in developed countries was associated with a decrease in the price of scrap steel.1 This increase in scrap availability and decrease in price created an opportunity for growth of scrap-based steel-making. With lower capital costs than an integrated mill, minimills based upon electric-arc furnace melting (EAF) of scrap were able to establish a cost advantage for the production of certain steel products.Figure 3 shows the proportion of crude steel production by process in the United States over the last 50 years.

The development of ultra-high-powered electric-arc furnaces and reliable billet and bloom continuous-casting machines provided a low-cost route for the production of lower quality steel long products, such as reinforcing bar and structural steels. As a result, integrated steel producers have been completely displaced from this low-end segment of the steel market in developed countries. This has allowed integrated producers to focus on the production of high-quality plate and thin-gauge flat products. The quality of steels produced via EAF is restrained by the level of metallic residuals such as copper, nickel, and tin, in the scrap metal charge and dissolved gasses such as hydrogen and nitrogen, which are contained in the scrap and picked up during processing. At very low levels these contaminants can significantly degrade the physical properties of many steel grades. However, continuous improvements in EAF process control and the use of ore-based scrap substitute materials such as direct reduced iron, hot briquetted iron, and pig iron to dilute tramp elements in scrap, have significantly increased the product quality range. Improved chemistry control and the successful implementation of thin-slab casting by Nucor has demonstrated that EAF producers can also be competitive in producing quality flat products as well. The continued expansion of EAF steelmaking for the production of higher quality steel products is projected to continue.10 However, this expansion will require continued technological development of the basic process of electric furnace steel-making. More than 40% of steel produced in the United States is produced by EAF, and that figure is expected to rise to 50% by 2010.

In the last 30 years, a number of major technical modifications of the electric arc furnace have dramatically improved the efficiency and productivity of the process. Up to the present, the primary focus of electric furnace development has been to increase productivity and energy efficiency by decreasing tap-to-tap time. Large heat losses occur while the scrap pile or liquid steel is at high temperature. Greater energy efficiency is achieved when the rate of energy input is increased and the time at temperature is decreased. As a result, many of the developments in EAF steelmaking have focused on increasing the net energy density that the furnace is capable of delivering.1 The development of foamy slag practices, whereby the hot electrode(s) and plasma arc are enveloped in foamed steelmaking slag, has significantly improved EAF performance. This practice protects the furnace roof and side walls from radiation and excessive heating, helps to stabilize the arc, and increases heat transfer to the steel, thus allowing furnace operators to run at much higher rates of power input.

Most modern electric furnaces also use a combination of oxy-fuel burners, pulverized coal injection, and oxygen injection to supplement electrical energy input. For modern EAF operations, 35% of the energy input is in from chemical energy sources.3 Recently, additional chemical energy has been recovered via post-combustion reducing of product gases by the controlled injection of additional oxygen into the furnace above the slag. In the most modern furnaces, oxygen injected to combust pulverized coal injection and carbon charged into the furnace in scrap steel, direct reduced iron, pig iron, coke or coal can be as high as 40 Nm3/ton. For furnaces with post-combustion systems, the oxygen usage may be as high as 70 Nm3/ton.1 At these very high rates of oxygen usage, significant additional heat energy is released by the exothermic oxidation of iron at high temperature. The additional heat input is gained at the expense of yield, due to the loss of iron as iron oxide in the slag. As a result, slag chemistry control and yield will become a focus of future developments in process control. Figure 4 shows the progress of EAF steelmaking in the last 30 years with respect to several key performance indicators.

The large quantities of hot combustion product gasses generated in the modern EAF have led to the development of several novel scrap preheating systems, whereby the heat energy of the exhaust gas is used to preheat scrap prior to melting. Between 10–30 percent of the energy input into an EAF can leave the furnace with the hot exhaust gas. Theoretically, 10 kWhr/ton of energy can be saved for every ~50 8C of preheating of the scrap charge.2 In practice, capture of the heat from furnace exhaust gas has been problematic, primarily due to emissions control complications. Until recently, relatively low energy prices have made the economics of scrap preheating marginal, particularly in cases where the efficient heat transfer could not be achieved. The Fuchs shaft furnace, the Consteel process, and the Nippon Steel/Davy-Clecim twin shell electric arc furnace concept are some examples of scrap preheating systems that are currently in commercial use. Several processes are under development that allow for continuous preheating, feeding, and melting of scrap.1

NEW TECHNOLOGIES FOR IRON AND STEELMAKING

Worldwide, direct-reduction capacity via existing gas-based technologies is likely to increase in order to support the expansion of EAF steelmaking to new, high-quality products. However, the blast furnace is likely to remain the backbone of worldwide iron production for several decades. Current fluidized bed and shaft furnace direct-reduction processes rely on natural gas as the primary reductant and source of heat for the reaction. One exception is the hydrogen-based Circored process. A Circored hot-briquetted iron plant in Trinidad produces reduced iron using byproduct hydrogen from the local petroleum industry. In regions where an abundant and inexpensive source of natural gas (or hydrogen) exists, gas-based direct reduction of iron followed by melting in an EAF can provide a cost-competitive alternative to quality steel products. However, in areas where low cost natural gas is not available, coal-based iron reduction processes will have an advantage. The efficiency and productivity of modern large-capacity blast furnaces will be difficult to surpass. However, high capital requirements make it unlikely that any new blast furnaces will be built in developed countries in the near future. Nevertheless, the shut-down of aging coking operations and older, smaller blast furnaces will force the industry to pursue one or a combination of three options:

Adopt or develop an entirely new process(es) for the production of hot metal or steel to complement or replace the blast furnace

Figure 5. Productivity at AK Steel’s Midland No. 3 blast furnace from June 1987 to August 1996. Adapted from Rabold and Hiernaux.18

There are several methods by which a limited supply of hot metal could be stretched by increasing scrap utilization. Process optimization of current oxygen steelmaking technologies will result in small improvements in yield by reducing both the iron content and total volume of slag produced. Scrap preheating and improved post combustion in conventional oxygen steelmaking vessels could be used to increase the scrap usage in these processes. Entirely new oxygen steelmaking furnace designs have been proposed, such as the energy optimizing furnace11, which makes use of high rates of post combustion, additional fossil fuel additions, and elaborate scrap preheating to increase scrap melting to as high as 70% during hot metal refining. Alternatively, direct hot metal addition and increased oxygen usage in a conventional EAF can dramatically decrease the electrical energy requirements per tonne of steel. The latter option allows the steelmaker to produce steel using anywhere from 20% to 100% scrap, producing the entire range of steel qualities with respect to residual content. Such a hybrid EAF-OSM process offers a great deal of process flexibility using proven and well-understood processes.

One limitation of stretching hot metal through significantly increased scrap utilization is related to the control of residual elements. As mentioned earlier, the quality of steels that can be produced via conventional EAF steelmaking is currently limited by control of residual metallic tramp elements in scrap and dissolved gasses. The increased production of steel long products via EAF steelmaking has resulted in a general decline in the quality of #2 and heavy melting scrap steel. If scrap is used in increasing quantities for the production of all steels, levels of residual elements can be expected to rise in the entire scrap supply. One solution to this dilemma is an economical process by which residual tramp elements can be removed from scrap, thus upgrading the scrap quality. Several processes have been demonstrated on laboratory and pilot scales12–17 that have been successful in removing certain tramp elements. However, in each case, unfavorable economics have prevented widespread commercial implementation.

One alternative to removing metallic tramp elements is to reduce their deleterious effects on steel properties. Most metallic residuals reduce steel hot strength and hot and cold ductility by segregating to and weakening grain boundaries. Tolerance to such chemical impurities could be improved through the design of alloys in which these elements were tied up in heterogeneously nucleating second-phase particles, which might not have the same negative effect on steel properties. Also, new near net shape casting processes, which will be described in following sections, may dramatically reduce the overall effect of residual elements for two reasons. As its name implies, near net shape casting describes solidification processes by which steel is cast in dimensions near to the specifications of the final product.

Although hot reduction at some level may still be required for microstructure control, near net shape casting significantly reduces the dimensional forming requirements of hot reduction processes and may reduce problems such as hot tearing. Even more importantly, near net shape casting processes for the production of thin gauge steel involve much higher rates of heat removal, solidification, and cooling than conventional casting or thin-slab casting processes. As a result, microstructural evolution in strip cast materials is fundamentally different from conventionally processed materials. Macro-segregation in strip-cast steel is significantly suppressed. Experiments have shown that a wide range of properties can be achieved from a single steel chemistry entirely through variation of casting speed and solidification and cooling rates. This has significant implications for the future of residual element control in steelmaking.

Industry leaders continue to demonstrate the significant potential for increased productivity of the blast furnace.18Figure 5 shows the production rate at AK Steel’s Middletown No. 3 blast furnace over a ten-year study period.

For blast furnace production to continue into the future even at current levels in the United States and other developed countries, continued progress must be made on reducing the coke rate of furnaces through coal injection. Significant progress has been made in evaluating the benefits of oxygen enrichment of the hot blast. Industrial trials are in progress to evaluate an oxy-coal injection system, which promises to allow for complete combustion at elevated coal injection rates.19 New, environmentally acceptable and economically feasible processes for new or replacement coke production capacity should be evaluated. If less coke is to be used in the blast furnace, the mechanical property requirements of the coke that is used will become more critical to maintaining permeability and stable furnace operation. If antiquated coke production methods cannot produce material of the required strength, exporting the environmental problems of coke production to developing countries with less stringent regulations may become functionally as well as socially unacceptable.